A prior art butterfly valve consists in a cylinder ring with shoulders for mutually positioning two pipework ends disposed end-to-end. An O-ring is disposed between the two pipework ends around the outside surface of the ring.
The inside diameter of the circular ring includes a narrowing that forms a shoulder. The shoulder narrows the nominal diameter of the duct. Two circular half-flaps are disposed inside the ring to pivot about a diametral axis. In the closed position, the two half-flaps bear against the shoulder and they are urged towards an open position by a helical torsion spring threaded over the axis. When no force is applied thereto, both half-flaps are therefore open and together they form a V with the extent to which the two branches of the V can move together being limited by a metal tab formed on each of the half-flaps and bearing against the wall of the duct.
Such a butterfly valve is effective in preventing a "hammer" shock wave from propagating or for preventing particles being swept up by too great a flow of gas; however, the use of a spring prevents the lifetime of the mechanism being very long if the flaps are moved frequently. In the event of the spring breaking, the valve no longer opens and there is a severe danger of damage to items situated downstream from the device. Under such circumstances, this cannot be accepted.